For those of you who have read Blake Snyder’s screenwriting guide, Save the Cat!, you know how important it is to do something early on in your script to make the audience care about your protagonist. For those of you who haven’t read Save the Cat!, you should seriously stop what you’re doing and go read it right now. I don’t agree with every single point in this book and it does tend to oversimplify things a bit, but it contains so much practical advice that any aspiring screenwriter should give it a look. Go ahead and read it now – I’ll wait.
Okay, now that you’ve read Snyder’s book, you know that the specific example that he gave about how to make a protagonist sympathetic was from the movie “Alien” where the Sigourney Weaver character, Ripley, is portrayed as a cold-hearted bitch. Since that type of character tends to be unsympathetic, screenwriter Dan O'Bannon did something early on in the story to force the audience to like her – he had her save a cat. I think that might have something to do with the title of Snyder's book, but don’t quote me on that. (Smiley face. Or wink. Or whatever.) The point is that anyone who goes through the trouble to rescue a cat can’t be all bad, so we develop a bond with Ripley that will last throughout the rest of the story. It's fun to manipulate the audience like this!
Another good example of a character who is sympathetic, despite being completely unlikable, is Melvin Udall, the Jack Nicholson character in “As Good As It Gets.” (N.B. – The first time I wrote that sentence, I accidentally typed the word “unlickable” instead of "unlikable." It still made sense, but that wasn’t the point I was trying to get across. Moving on.) He’s not just unsympathetic, he’s staggeringly cruel and thoughtless to every single person he meets. In order to get the audience to sympathize with Melvin, screenwriters James L. Brooks and Mark Andrus take several specific steps: First, they show that Melvin actually has a mental disorder, which lets the audience know that his cruelty is not entirely within his control. Second, we find out that Melvin is actually a best-selling romance author, which means that somewhere behind that evil façade, lies the heart of a romantic. Third, Melvin does something extraordinarily kind for his waitress/love interest, Carol, by getting her son the medical attention that he needs. And finally, Brooks and Andrus show Melvin being mean to a dog at first, then bonding with that dog later.
The big takeaway is that if you want to make a character sympathetic, have him do something nice for an animal; if you want to make him unsympathetic, have him do something mean to an animal. See how easy screenwriting is?